1. Field of the Invention
The present invention relates to an angle-calculation apparatus and angle-calculation apparatus method for optical encode, especially to an angle-calculation apparatus and angle-calculation apparatus method for three-phase optical encoder.
2. Description of Prior Art
The AC servomotor generally comprises an optical encoder wheel to sense angle information of a rotator, this angle information can be used to determine an electromagnetic field for driving stator current. Therefore, the speed of the AC servomotor can be precisely controlled
FIG. 1 shows the schematic diagram of a prior art AC servo motor. The angular position of rotor in a motor 10 is detected by an optical encoder 12 and processed by a signal processing unit 20 to obtain angular information. The angular information is processed by a speed estimation unit 14 to obtain an estimated motor rotational speed. A speed controller 30 receives the estimated motor rotational speed and a speed command to control a controller module 32 and an IGBT module 34 in order to generate a motor speed control signal. The motor speed control signal can be used to precisely control the rotational speed of the motor 10.
The noise of the AC servomotor can be advantageously reduced if the optical encoder wheel can provide higher resolution. The conventional ways to enhance resolution for grating type optical encoder wheel includes: 1. Increasing the mark number on the optical encoder wheel. 2. Fine division by electronic skill. 3. Using different optical principle. The first method has limited effect because manufacture difficulty and diffraction phenomenon. The second method is more feasible because the mechanical structure does not need immense change. The third method needs to change the original architecture, such as using laser diode. Moreover, different optical design such as diffraction or interference are involved to enhance resolution.
The fine division for existing optical encoder includes following four types. 1. The fine division mechanism is incorporated into the optical encoder such as GPI 9220, DRC 25D, RSF MS 6X series. 2. Standalone product, such as RENISHAW RGE series, HEIDENHAIN EXE 605 and SONY MJ100/110, MJ500/600/700 Series Interpolation Module. 3. The fine division mechanism is integrated into controller card or other products such as MM1200-PC/104. 4. The fine division mechanism is integrated into motor such as Fanuc, Mitsubishi. The fine division skill can provide 4-2048 times enhancement or more, which depends on the quality of original signal and signal compensation skill.
The fine division method can be classified to phase fine division and amplitude fine division. The A, B output signals from an encoder are quasi-sinusoidal signal and can be expressed as:A=U0 sin αB=U0 cos α
U.S. Pat. No. 6,355,927 discloses an amplitude fine division method, wherein A, B output signals of different amplitudes are under subtraction operation and processed by logic gate to achieve fine division.
As the speed of DSP and MPU is increased, the fine division scheme can be implemented by ADC with the help of DSP and MPU. The signals are actively or passively adjusted for higher resolution. FIGS. 2 and 3 show block diagram and flowchart for an implementation of electronic fine division. As shown in FIG. 2, the output angular information Sin ω and Cos ω of the encoder 12a are processed by an ADC 22a to convert into digital signals SIN X and COS X. The output angular information Sin ω and Cos ω of the encoder 12a are processed by a phase digitizer to obtain a digital signal M. The digital signal M is processed by the high speed signal processing portion 26a to generate a turn number signal N and a phase signal PH to provide quadrant and comparison information to the DSP 28a. With reference to FIG. 3, the DSP 28a determine the rotor angle θ according to the digital signals SIN X and COS X, the turn number signal N and the phase signal PH.
The etching technology for glass plate cannot satisfy the requirement of high resolution optical encoder. To obtain higher resolution, interpolation is performed to the quasi sinusoidal signals, which is produced by passing light through optical grating on the glass plate. In prior art angle-calculation method for motor rotor, sine signal and cosine signal (90 degree different to the sine signal) are used as source signals, and inverse function of tangent (Tan) or phase lock loop is used for angle calculation. However, full-amplitude portion is used for the sine signal and cosine signal. The source signals are required to be pure sinusoidal. The angle calculation is degraded when the source signals have harmonic component other than pure sinusoidal waves.
The sinusoidal signals with 90 degree difference are used to evaluate the impact of harmonics on interpolation angle:
      A    =                            U          0                ⁢        sin        ⁢                                  ⁢        θ            +                        1          8                ⁢        sin        ⁢                                  ⁢        3        ⁢        θ            +                        1          32                ⁢        sin        ⁢                                  ⁢        5        ⁢        θ            +                        1          128                ⁢        sin        ⁢                                  ⁢        7        ⁢                                  ⁢        θ              when      B    =                            U          0                ⁢                  sin          ⁡                      (                          θ              -                              π                2                                      )                              +                        1          8                ⁢        sin        ⁢                                  ⁢        3        ⁢                  (                      θ            -                          π              2                                )                    +                        1          32                ⁢        sin        ⁢                                  ⁢        5        ⁢                  (                      θ            -                          π              2                                )                    +                        1          128                ⁢        sin        ⁢                                  ⁢        7        ⁢                  (                      θ            -                          π              2                                )                    
The interpolation angle θ calculated by the inverse function of tangent
  θ  =            tan              -        1              ⁢          A      B      with fine division method shown in FIGS. 2 and 3 will have periodic error of ±2.5% because of harmonics. Therefore, the prior art angle-calculation method cannot achieve high resolution and has no robustness.